1. Field of the Invention
The present invention relates to an image processing apparatus, a tomography apparatus, an image processing method, and a storage medium.
2. Description of the Related Art
Tomography apparatuses that use radiation are widely used in clinical practice. Tomography is an imaging technique of obtaining multiple projected images by imaging the same object from various angles, and thereafter reconstructing a tomographic image of the object by filtered back projection or successive approximation using a specific reconstruction filter. In recent years, tomosynthesis in which reconstruction is performed using projected images obtained from a limited number of imaging angles has also gained popularity.
Since multiple projected images are captured in one operation for tomography, it is desired that the radiation exposure amount of the object is reduced, but noise included in the images increases because of the reduction of the imaging radiation amount. For this reason, providing higher-performance noise reduction processing in order to provide images with high diagnostic value even in low radiation exposure conditions has been a very important issue.
In order to address this issue, Patent Literature 1 (Japanese Patent Laid-Open No. 2003-180675) proposes a method of reducing the noise in a projected image before it undergoes reconstruction processing. In addition, Patent Literature 2 (Japanese Patent Laid-Open No. 2005-160544) proposes a method of reducing the noise in a tomographic image that has been reconstructed.
However, there are cases in the above-mentioned related art where the following problems occur. For example, depending on the type of reconstruction filter in the technique disclosed in Patent Literature 1, there are cases where an image smoothing effect obtained by the reconstruction filter and an image smoothing effect obtained by the noise reduction processing performed on the projected image are obtained redundantly and thus an image is not obtained.
Also, in the technique disclosed in Patent Literature 2, with a technique such as tomosythensis, in which reconstruction is performed based on an incomplete projection set using projected images obtained from a limited number of angles, the CT value of the tomographic image loses its physical significance. Because of this, there are cases where it is difficult to estimate the noise component included in the tomographic image and noise reduction cannot be performed.